1. Field of the Invention
The invention relates to the production of synthetic gas, and more particularly to a method and a system for producing synthetic gas from biomass by high temperature gasification.
2. Description of the Related Art
Biomass, an organic matter generated by plants through photosynthesis, has wide sources and large available quantity. It can be transformed into clean gas or liquid fuel for power generation and producing industrial raw materials and chemical products. As energy it is clean and renewable with zero emission of carbon dioxide.
There are many methods for transforming biomass into clean gas or liquid fuel, among which biomass gasification technology can adapt to a variety of species and has good expansibility. The gasification of biomass is a thermochemical process, i.e., biomass reacts with a gasification agent (such as air, oxygen, vapor, carbon dioxide, etc.) under high temperature to produce a mixed gas consisting of carbohydrate containing carbon, hydrogen, and oxygen. The mixed gas is named synthetic gas. The components of the synthetic gas are decided by the species of used biomass, the type of the gasification agent, the reaction conditions, and the structure of a gasifier used therein. The objectives of gasification is, on the one hand, to minimize the consumption of materials and the gasification agent, as well as the tar content in the synthesis gas, and on the other hand, to maximize the gasification efficiency and the efficiency of carbon conversion, as well as the active ingredient (CO and H2) content in the synthesis gas. The objectives are decided by the type of the used gasifier, the type of the gasification agent, the particle size of the biomass, the gasification pressure and temperature, and moisture and ash of the biomass, etc.
Conventional gasifier is in the form of a fixed bed, a fluidized bed, or an entrained flow bed. The fixed bed has a simple structure and flexible operating mode, and is easy for practice. Solid materials have a long retaining time in the bed, the efficiency of carbon conversion is high, the operating load is wide (changeable between 20 and 110%). However, the temperature in the fixed bed is nonuniform, the heat exchange effect is poor, the synthetic gas has a low heating value, and a large amount of tar is produced.
The fluidized bed is convenient for material addition and ash release, and the temperature is uniform and easy for adjustment. However, it is sensitive to the characteristics of raw materials. If the adhesion, thermal stability, moisture content, or ash melting point of raw materials changes, the operation will become abnormal. Furthermore, the synthetic gas has a large amount of tar. Since a large amount of tar is produced in the fixed bed and the fluidized bed, a tar cracking unit and purification equipment must be installed, which results in a complicated process.
The entrained flow bed has a high and uniform operating temperature, good amplification characteristics, and particularly suitable for large-scale industrialization. Tar is cracked completely. However, the entrained flow bed has a strict requirement on particle size of raw materials. Based on current grinding technology, there is no way to grind biomass having much cellulose to a size suitable for the entrained flow bed. So the entrained flow bed cannot be used for gasification of biomass. Nowadays, tar cracking and pretreatment of biomass prior to gasification are tough problems for the development of biomass gasification.
Chinese Patent Application No. 200510043836.0 discloses a method and a device for gasifying low tar biomass. The method includes pyrolysis and gasification independently, and biomass is transformed into synthetic gas containing low content of tar. In the method, pyrolysis gas and charcoal experience incomplete combustion in the gasifier at around 1000° C., and tar is cracked under high temperature. Although the tar content is decreased greatly, a lot of charcoal is consumed, resulting in a low content of CO produced in the subsequent reduction reaction and a high content of CO2 in the synthetic gas. Secondly, due to a low temperature in the combustion reaction, the average temperature in the reduction zone is less than 700° C., and thereby the yield of effective synthetic gas (CO and H2) is decreased significantly (about 30%). Thirdly, the ash and unreacted carbon residue from the reduction reaction is directly discharged, resulting in a low carbon conversion rate. Finally, the gasifier used in the method is in the form of a fixed bed, since the reduction reaction absorbs heat, the temperature difference between the top and the bottom (the top is about 1000° C. and the bottom is about 500° C.) of the bed is huge, which is an inherent disadvantage of fixed bed.
U.S. Pat. No. 6,863,878B2 discloses a method and a device of producing synthetic gas with carbon-containing materials. The method includes carbonization (or pyrolysis) and gasification independently. In the method, the carbonization temperature is controlled less than 450° F. as so to reduce the tar content resulted from pyrolysis. However, during carbonization stage, solid products are not ground prior to transporting to the reaction coils of the gasifier, which will affect the speed and degree of gasification reaction. Secondly, since the gasification reaction happens in the reaction coil, a large amount of transport gas is needed, but the transport gas will take away a lot of heat during transporting, and thereby the gasification efficiency is low, the temperature is nonuniform, and the subsequent waste heat recovery system is massive. Thirdly, it is not economic that newly-produced synthetic gas is used to provide heat for gasification and carbonization. Fourthly, combustion products (mainly CO2 and H2O) are directly discharged and not fully utilized, resulting in low gasification efficiency. Finally, the ash and unreacted carbon residue in the synthetic gas are also discharged directly, resulting in low carbon conversion rate.
Chinese Patent Application No. 200610124638.1 discloses a method of producing synthetic gas from biomass by combined cycle high temperature gasification. The method includes carbonization and high temperature gasification. However, in the method, heating by the gasifier or cycled synthetic gas has hidden danger, the heating rate of pyrolysis is very slow, material consumption is high, and thereby the total gasification efficiency is low. Secondly, the charcoal powder transportation system (two-stage ejecting) is complicated, for high temperature gasification system, the synthetic gas for charcoal powder transportation is something like inert gas, so more oxygen and effective synthetic gas are consumed, and the gasification efficiency will be decreased by about between 5 and 10%. Furthermore, high pressure of charcoal from the carbonization furnace is directly transported into a high pressure milling machine after cooling without decompression, which is very difficult to achieve in industry.
From the above mentioned methods, conventional gasification, whether from biomass or from solid carbon-containing materials, cannot produce synthetic gas with high efficiency and low cost. Although the technology of independent pyrolysis and gasification can adapt to a variety of biomass and reduce the content of tar in synthetic gas, shortcomings such as nonuniform temperature, large investment in equipment for waste heat recovery, high material consumption, low gasification efficiency, and low carbon conversion rate limit the application of biomass gasification in industry. Particularly, there is no effective method for gasifying biomass applied to an entrained flow bed.